The present invention relates to an exhaust gas cleaner capable of efficiently removing nitrogen oxides from an exhaust gas containing substantially no fine carbonaceous particles, which is regarded as an oxidizing atmosphere, and a method of cleaning an exhaust gas using such an exhaust gas cleaner.
The nitrogen oxides (hereinafter referred to simply as "NOx") contained in the exhaust gas discharged from internal engines of automobiles, etc., various external engines, industrial combusting equipments, etc. are causing environmental problems, because the NOx has been known as a main cause for acidic rain.
As methods for removing the NOx from the exhaust gas, there has been known to reduce the NOx discharged from gasoline engines to a harmless N.sub.2 in the presence of a three-component catalyst, or to react the NOx discharged from the industrial exhaust gas with NH.sub.3 as a reducing agent in the presence of an oxide catalyst.
However, in general, in an atmosphere of high-oxygen concentration, the reduction reaction of the NOx is less likely to proceed without adding a reducing gas. For instance, in the exhaust gas of gasoline engines, etc., under the conditions in which the exhaust gas has a high oxygen content and thus a high air-fuel ratio (on the lean side), it is difficult to reduce the NOx in the presence of a three-component catalyst. Therefore, an attempt has been made to reduce the NOx in the presence of a proper catalyst by using fine carbonaceous particles (hereinafter referred to simply as "particulate matter") as a reducing agent. However, since the amount of fine carbonaceous particles contained in the exhaust gas of the gasoline engines is extremely small, this method cannot practically be used.
Recently, an intense research has been made on catalysts capable of efficiently removing NOx and methods of removing NOx by using such catalysts.
For instance, Japanese Patent Laid-Open No. 1-94946 discloses a highly active catalyst for decomposing nitrogen oxides, which catalyst comprises an aluminosilicate zeolite having a particular lattice surface distance and supporting copper (I) or copper (II) and ammonia at a molar ratio (ammonia/copper) of 0.2 or more.
Japanese Patent Laid-Open No. 63-100919 discloses a method of removing nitrogen oxides from an exhaust gas by bringing the exhaust gas containing nitrogen oxides into contact with a copper-containing catalyst in the presence of a hydrocarbon in an oxidizing atmosphere.
However, the conventional catalysts and methods exemplified above suffer from poor efficiency and durability when the exhaust gas has a high oxygen concentration, namely, in an oxidizing atmosphere. Accordingly, they are not advantageous for an actual use.